Please be assured what you describe is not a standard Firefox feature.

It may have been easier to see what you have done to Firefox if you had included full troubleshooting information.

• Have you set MSN as a homepage
  • see How to set the home page
• Or maybe set it to open the MSN site by including that in the shortcut used to start Firefox
• Another thing you may have done is install adware or a toolbar
  • see Remove a toolbar that has taken over your Firefox search or home page

Here's a less likely possibility: some malware edits your Firefox shortcut to include a specific URL. To check for that, right-click the shortcut, choose Properties, then check the "Target" to make sure it doesn't have any URLs after firefox.exe.

Also if needed, use the searchreset tool and delete a possible user.js explained in the linke/articles below.

• Search Reset Tool.

• https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save#w_user-js-file-overriding-preferences